How "green" are 15-year-olds? To what extent do they understand environmental issues? What are the sources of their information about the environment? Can they apply scientific principles and evidence to the world around them? This paper reports that across OECD (Organisation for Economic Cooperation and Development) countries, around one in five students is consistently able to identify, explain and apply scientific concepts related to a variety of environmental topics. In Canada, Finland and Japan, over a third of 15-year-olds have high levels of environmental literacy. Students acquire most information about environmental issues from school, although only a minority of students learns about these issues in stand-alone environmental science courses. Thus, schools are a crucial source of information on environmental issues for students. While in most countries only a minority of schools has courses dedicated to the environment, the issue is often discussed as part of other core curricula, and many schools offer out-of-school activities that focus on the environment.